Flash it to an USB drive: dd if=path-to-iso.imgof=/path/to/usb-drive
bs=8M

Boot from the USB drive (choose the EFI option in the boot menu)

Find your EFI partition using fdisk -l. You will recognize it by the
partition type, which is EFI System.

Create a mountpoint for the partition: mkdir /efi

Mount the partition: mount /path/to/efi-partition /efi

Make sure that the now mounted /efi directory contains the EFI
subdirectory.

(Re-)Install systemd-boot: bootctl --path=/efi install

Reboot

In my case, everything worked again.

Note that if you were a Gummiboot user, you should probably read through
https://wiki.archlinux.org/index.php/Systemd-boot. Among other interesting
things, it's important to know that updates are not applied automatically via
post_install anymore. Instead, run bootctl update after updating
systemd-boot.